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NAOSITE: Nagasaki University's Academic Output SITE Title Power efficiency improvement of the composite resonant DC-DC converter Author(s) Kato, Hisatsugu; Ishizuka, Yoichi Citation Renewable Energy Research and Applications (ICRERA), 2012 International Conference on, Article number 6477425; 2012 Issue Date 2012 URL http://hdl.handle.net/10069/32475 Right © 2012 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. This document is downloaded at: 2016-10-22T20:26:53Z http://naosite.lb.nagasaki-u.ac.jp Power Efficiency Improvement of the Composite Resonant DC-DC Converter Hisatsugu Kato* and Yoichi Ishizuka† * Power Electronics Engineering Department Tabuchi Electric CO., LTD Osaka 532-0007, Japan Tel: +81-6-4807-3535, Fax: +81-6-4807-3534 E-mail: [email protected] † Nagasaki University, Nagasaki, 852-8521, Japan E-mail: [email protected] Abstract—This paper deals with the power efficiency improvement of the composite resonant DC-DC converter, which is developed to be applied to the power conditioner of the photovoltaic generation system. To improve the power efficiency, following three approaches are taken: 1) optimum design of the transformer 2) use of the voltage doubler rectifier. 3) reduction of the winding loss of the transformer. The maximum power efficiency of 98.0% can be realized. Keywords-component; power efficiency improvement, composite resonant DC-DC converter, optimum design, voltage doubler rectifier, leakage flux I. INTRODUCTION Of all the energy used all over the world, the fossil energy from such as oil, coal and natural gas is the largest in number, which emits the carbon dioxide that causes global warming.. As a result, abnormal weather, the rise of a sea surface and so forth are predicted. To solve this problem, great attention to the new energy resources such as the photovoltaic cell, fuel cell and wind force are are given. This paper deals with the power efficiency improvement of the composite resonant DC-DC converter[1-4], which is developed to be applied to the power conditioner of the photovoltaic generation system. To improve the power efficiency, following three approaches are examined: (1) Optimum design of the transformer with reducing the size and weight of the transformer (2) Use of voltage doubler rectifier (3) Reduction of the winding loss caused by the leakage flux in the vicinity of the air gap of the transformer II. CIRCUIT CONFIGURATION Fig.1 shows the proposed current resonant DC-DC converter with the voltage doubler rectifier, in which the current and voltage resonant circuits are employed. In this figure, TR1 and TR2 are main switches of MOSFETs. CTR1 CR T TR1 Cd DTR1 CTR1 D1 + Co LP LS D2 R Eo Ed TR2 DTR2 Fig.1 Half-bridge current resonance DC- DC converter with the voltage doubler rectifier. and CR are the voltage and current resonance capacitors, respectively. LP and LS are inductances of the primary and secondary windings of the transformer T. The voltage doubler rectifier is composed of the diodes D1 and D2, and capacitances Cd and Co. The MOSFET switches TR1 and TR2 are turned-on and turned-off alternatively. There exists the short dead time between the on-times of TR1 and TR2. The photovoltaic generation system has the output capacity of 2 kW. The DC-DC converter requirements are 80-450V input voltage, 396V output voltage and 2150W maximum input power. III. POWER EFFICIENCY IMPROVEMENT A. Optimum design for the transformer (Reducing the transformer’s size and weight.) To realize both of improvement of power efficiency and downsizing of the DC-DC converter, optimization of transformer is performed. The comparison of specifications with conventional one is summarized in Table 1. In Table 2, the compared results for categorized power consumption of the optimized transformer and conventional one are summarized. From these results, the volume and weight of 57% of the optimized trans-former can be reduced, compared to the conventional one. By downsizing Table 1 Specifications of DC-DC converter of the optimized transformer and conventional one. Transformer form Air gap Diameter and turn of Primary winding with litz wire Diameter and turn of Secondary winding with litz wire Resonant Capacitor The optimized PQ50 1.2mm Conventional PQ71 1.5mm 0.06mm / 1350 turn 0.12mm / 504 turn V:200V/div V:200mA/div H:200ns/div 0V、0A 0.06mm / 810turn 0.12mm / 252turn 1.16µF 0.88µF Recovery current Table 2 Power consumption and power efficiency comparison of the optimized transformer and conventional one. Optimized transformer(W) MOSFET ( High + Low ) conduction loss MOSFET ( High + Low ) switching loss Rectifier diode loss(D1 and D2) Rectifier diode loss ( D1, and D2) Winding copper loss (Primary) Winding copper loss (Secondary) The other winding losses Loss of EMC filters Other losses Power efficiency( 250V、 2150W) Conventional transformer (W) 11.0 10.0 1.2 0.8 12.0 11.2 0.7 0.9 4.5 2.3 6.5 2.2 13.7 5.5 3.0 97.3% (Total loss 58.1W) 24.3 5.5 3 97.2% (Total loss 60.2W) of transformer, winding copper loss is increased, but otherwise the other winding losses, including iron loss and proximity effect loss, are become almost a half. As the result, downsizing of the transformer is realized with the increasing of 0.1% power efficiency. Precise descriptions are described in later. B. Employing the voltage doubler rectifier. To compare the recovery loss of voltage doubler rectifier and center tapped rectifier, the requirements are settled with 210V input voltage, 180V output voltage and 1kW output power. Figure 4 shows the conventional half bridge current H:50V/div V:500mA/div V:200ns/div Repetitive peak reverse voltage Fig. 3 Center tapped rectifier. resonance DC-DC converter with the center tapped rectifier. In the experiment, FRD of the maximum reverse voltage 600V is used as a rectifier diode. The requirements are maximum requirements to avoid over avoid maximum reverse voltage of FRD. In Figs. 2 and 3, the waveforms of voltage and current of the voltage doubler and the center-tapped rectifier are shown, respectively. Each of the recovery loss is 0.42W and 0.94W, respectively. The rectifier loss is reduced in half. The recovery loss of diodes is reduced by employing the voltage doubler rectifier in the secondary side as shown in Fig. 1. By the capacitor Cd in the voltage doubler, the reverse current loss is reduced. Another advantage of the voltage doubler rectifier is it can be able to treat more power than the center tapped rectifier. From the experimental result, the maximum output power was 2150W with 250V input voltage, 400V reverse voltage. Also, the recovery loss is almost 0.4W. The reverse voltage is clumped to regulated voltage between output voltage and secondary voltage of transformer. Therefore, the loss occurred by the oscillation is suppressed. Same result is realized with full bridge rectifier, but the number of diodes and heat sink are increased. The both of cost and difficulty of the circuit implementation are increasing. CR TR 1 0V、0A T D1 DTR1 CTR1 Co + LP LS R E d Recovery TR DTR D2 Repetitive peak reverse voltage Fig. 2 Voltage R titidoublerkrectifier. lt Fig. 4 The half bridge current resonance DC-DC converter with the Center tap rectifier. Primary Winding Temperature rise (deg) 140 120 Air gap 100 80 60 40 20 0 0 0.02 0.04 0.06 0.08 0.1 0.12 Strand diameter of ritz wire (mm) Fig.5 Temperature rise vs. strand diameter of the litz wire C. Reduction of the winding loss by the leakage flux in the vicinity of the air gap of the transformer The leakage flux in the vicinity of the air gap of the transformer intersects with the winding. Then eddy-current loss and loss due to proximity effect are caused. If the switching frequency increases with the rise of the input voltage, these losses increase. It is presumed as described above. In order to solve this problem, as shown in Fig. 5, temperature rise of winding is improved by making the strand diameter of the litz wire small. That is, the loss by winding is reduced and the power efficiency is improved. The magnetic analysis using simulation was performed. The simulation was performed to avoid the memory overflow with simplified settings that bundle litz wires are unified to 0.8mm wire. Figure 6 shows simulation results. Each figure shows the magnetic field variation at θ=0 deg., θ=45 deg. and θ=90 deg. The applied input alternative current is cosine waveform, that peak value is at 0 deg. , Typically, winding loss is simply categorized to skin depth loss, eddy current loss and proximity effect loss. The simulation results are analyzed with this categorization. The diameter of litz wire using in experiments is 0.06mm. 0.06mm of the diameter size is decided as the minimum limit of mass productivity. The estimated skin depth is calculated 0.19mm with switching frequency 120 kHz. Therefore, the skin depth effect and loss can be ignored. Also, the test is performed with putting litz wire near to air gap. From the result, there was any generating heat in litz wire, that is, eddy current loss is able to be ignored. After all, it can be thought that the most of winding loss is occurred by proximity effect. Secondary Winding (a) θ=0deg. (Peak Primary Current) (b) θ=45deg. (c) θ=90deg. (Zero Primary Current) Fig. 6 The magnetic analysis results. IV. POWER EFFICIENCY Fig. 7 shows the power efficiency characteristics of the composite resonant DC-DC converter. It is seen that the power (T) REFERENCES 99 Input voltage 420V DC Power efficiency (%) 98 97 96 Input voltage 300V DC 95 Input voltage 250V DC (rated) 94 93 Input voltage 150V DC 92 Input voltage 80V DC 91 90 0 500 1000 1500 2000 2500 Output power (W) Fig.7 Power efficiency of the proposed composite resonant DC-DC converter. efficiency is very high and that the maximum power efficiency of 98.0% can be realized at 300V DC of the input voltage. Then the output voltage is 380 V DC. The high power efficiency over 97.0% can be realized from 150V DC to 420V DC of the input voltage and the high power efficiency over 97.0% can be realized at 250V DC (rated) of the input voltage and from 400W to 2150W of the input power. V. CONCLUSION From the above discussion, the high power efficiency is obtained using the proposed half.-bridge current resonance DC-DC converter. The maximum power efficiency is very high and 98% when the input voltage is 300 V DC. The power efficiency of 97% is realized at 250 V DC (rated) of the input voltage and from 400W to 2,150 W of the input power. The improvement has been done with: (1) Optimum design of the transformer with reducing the size and weight of the transformer (2) Use of voltage doubler rectifier (3) Reduction of the winding loss caused by the leakage flux in the vicinity of the air gap of the transformer [1]Daisuke Yodhitomi, Junichi Itoh, Katsuya, Hirachi, “Relationship between Leakage Inductance and Surge Voltage on Isolated DCDC Converter”, Journal of the Japan Institute of Powere Electronics, vol,37,pp.66-80,2011 [2]Koichi Morita,”Ultra Low-noise switch-mode Power Supply”, Journal of Japan Institute of Power Electronics, Vol.30, pp4-11, March 2004 [3]Hiroto Mizutani, Tomokazu Mishima, Yasuyuki Ooue, Yoshiki Fukumoto, Mutsuo Nakaoka,”Fundamental Analysis for LLC Resonant DC-DC Converter with a Reverse Multi-resonant LC Filter on Characteristics of DC Voltage Conversion Ratio”,Japan Institute of Power Electronics, Vol.37,pp168-174,2011 [4]C.F.JIN,T.Ninomiya,S.Tomioka,”Dynamic Characteristics of LLC Current-Resonant DC-DC Converter”, IEICE Technical Report, EE2008-35,pp.1-6,October 2008